Compact wide angle acoustic transducer

ABSTRACT

A transducer is provided for emitting and receiving acoustic waves, a method for operating and a method for producing the same. The transducer comprises a casing that forms a cavity. The casing comprises an excitation surface and an emitting surface, which is arranged opposite to the excitation surface. A transducer element is provided at the excitation surface, and an acoustic diffuser is provided at the emitting surface of the casing, wherein a diffusing structure of the acoustic diffuser faces the cavity, and wherein the diffusing structure is provided by an array of columns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/496,833, filed Apr. 25, 2017, which is a continuation ofInternational Application No. PCT/IB2016/052320, filed Apr. 25, 2016,the contents of each are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The current specification relates to an acoustic transducer forgenerating a wide angle, extended pressure distribution by making use ofdiffuse internal reflections.

BACKGROUND

The U.S. Pat. No. 4,156,863 discloses a wide angle, wide bandwidthtransducer which emits acoustic signals into a wide region by providingan array of many transducer elements that point radially outward along acurved surface.

SUMMARY OF INVENTION

By contrast, the wide-angle transducer according to the presentspecification makes use of one or more acoustic diffusers that areprovided within a cavity of a casing.

The present specification discloses a transducer for emitting andreceiving acoustic waves. The transducer is also referred to as “wideangle transducer”. In particular, the acoustic waves, which are emittedand received by the transducer, can be ultrasound waves that aretransmitted through a fluid across a closed or an open fluid conduit.

The transducer comprises a casing, which encloses a cavity. In otherwords, the casing forms or defines a cavity. The cavity of the casingcomprises a filling material such as a liquid or a gel, an epoxymaterial or other plastic material. The filling material provides acoupling from and to the surfaces of the casing. In particular, thefilling may fill out an interior space of the cavity essentiallycompletely. The interior space of the cavity is defined by the cavityand any elements that protrude into the cavity. For brevity, theinterior space of the cavity is also referred to as cavity where themeaning is apparent from the context or alternatively as “acousticcavity”.

If the filling material or transmitting medium is provided by a liquidor gel, the casing can be made watertight or liquid tight to ensure thatno air can be trapped or gel can leak. The casing of the wide-angletransducer comprises an excitation surface and an emitting surface,which is arranged opposite to the excitation surface. In particular, theexcitation surface and the emitting surface can be provided parallel toeach other and at opposite ends of the casing.

However, the emitting surface may also be tilted with respect to theexcitation surface. Furthermore, the excitation surface and the emittingsurface can be provided as plane surfaces. In other embodiments, theemitting surface or the excitation surface may be curved, for example toconform with a shape of round conduit.

In particular, an exterior portion of the excitation surface or theemitting surface can be provided as plane surface or as curved surface.The interior space of the casing in which the one or more diffusers areprovided can have a particularly simple shape, which is easy tomanufacture, such as a cuboid or a cylinder, in particular a circularcylinder. A cuboid is particularly suitable for fitting a diffuser witha rectangular base surface into the casing. In one embodiment, thecasing has the shape of a cuboid and the emitting surface and theexcitation surface are provided as opposite surfaces of the cuboid.

By way of example, the casing can be made from plastic material or itcan also be made from a metallic material. The transducer comprises atleast one ultrasound transducer element, which is coupled to or providedat the excitation surface of the casing. For example, the transducerelement can be inserted through the excitation surface and/or it can bewelded, soldered or glued to the excitation surface. By way of example,the traducer element or the transducer elements may be connected to theexcitation surface by press fit or by form fit. The transducer elementor the transducer elements are accessible from the exterior of thecasing and can be connected to electric cables for providing anexcitation signal to the transducer element or elements or fortransmitting a received signal.

An acoustic diffuser is provided at the emitting surface of the casing.A diffusing structure of the acoustic diffuser faces the cavity. Inother words, the diffusing structure protrudes into the interior spacedefined by the cavity.

For an even better diffusion of the sound waves within the cavity, asecond diffuser can be provided. In particular, the second diffuser canbe provided opposite to the first diffuser and such that a diffusingstructure of the second acoustic diffuser faces a diffusing structure ofthe first acoustic diffuser. The diffusing structures are facing eachother or point towards each other in the sense that there is a line ofsight between the diffusing structures.

In particular, the diffusing structures can be arranged relative to eachother such that respective mean directions, which are defined by thediffusing structures, point in opposite directions within apredetermined angle, such as 10 degrees, 20 degrees or 45 degrees. Forexample, two column type diffusing structures can be arranged such thatthe columns of the first diffusing structure point towards the columnsof the second diffusing structure within a predetermined angle, such as10 degrees, 20 degrees or 45 degrees. Specifically, the acousticdiffusers can be arranged such that respective base surfaces of theacoustic diffusers are aligned in parallel.

The respective alignments and the distance of the diffusing structuresfrom each other can be chosen such that a major portion of the soundemitted by one of the diffusers is received by the other diffuser.Thereby, an effective coupling between the diffusers is provided. Forexample, a distance between two column type diffusing structures can besuch that a minimum distance between the diffusing structures is no morethan 10%, 20% or 50% of a maximum height difference between the columns.

A base surface of the diffuser may cover the emitting surface or theexcitation surface essentially completely. Similarly, the respectivediffusing structure may cover the emitting surface or the excitationsurface essentially completely. Specifically, “essentially completely”may refer to 90% or more, 95%, or 99% or more of the emitting surface orof the excitation surface.

In particular, the diffusing structure can be realized by protrusions orby a superstructure on a base surface of the respective acousticdiffuser, such as walls or columns, which are aligned orthogonal to thebase surface or at an angle to the base surface.

The second acoustic diffuser is provided or attached to the excitationsurface such that a diffusing structure of the second acoustic diffuserfaces towards the cavity. The second diffuser can be of the same type asthe first diffuser or it can be of a different type. By way of example,the diffusers can be attached to the respective surface or to the casingby welding, gluing, screws, bolts, form fit or press fit.

By way of example, the excitation surface or the emitting surface can beprovided by a portion of the casing or by a portion of the casing andfurther elements that are attached to the casing. Furthermore, therespective acoustic diffusers can be provided at an internal portion ofthe respective excitation surface or at an internal portion of thecasing.

The casing may comprise openings for exchanging the liquid or gel andfor degassing purposes. Furthermore, the casing may comprise a lid onone side for the purpose of inserting or exchanging the acousticdiffuser. In one embodiment, the lid is glued or welded to the casing.According to another embodiment, the lid is fastened by screws. In thelatter case, a sealing or gasket may be provided, such as a seal ring orsimilar.

In particular, the diffusing structure of the acoustic diffuser or thediffusing structures of the acoustic diffusers may comprise a columntype diffusing structure. More specifically, the diffusing structure orthe diffusing structures may consist of or may be provided by a columntype diffusing structure. The column-type diffuser structure is alsoknown as terrain structure, wherein columns of the column type diffuserhave a statistically distributed height. In this case, the diffuser isalso referred to as column-type diffuser.

The statistically distributed height can be defined by a predeterminedalgorithm and the probability of a given height may depend on thelocation on the base surface, which can be specified by rectangularcoordinates x and y. Or, the probability of a given height may alsodepend on a distance from a location on the base surface of thediffusing structure or of the acoustic diffuser.

In general, the base size, the height of the columns and otherdimensions depend on the ultrasonic wavelength. According to oneembodiment, a base size of the columns of a column-type acousticdiffuser is between 0.5 square millimetres and 1.6 square millimetres.In particular, a column cross section can be rectangular or squareshaped and the size of the cross section can be between 0.75 mm×0.75 mmand 1.25 mm×1.25 mm. Furthermore, the column cross section can be thesame from the base to the top of the columns, whereby the base size isthe same as the cross section size.

According to a further embodiment, a base size of the columns of acolumn type acoustic diffuser is between 3 square millimetres and 5square millimetres. In particular, a column cross section can berectangular or square shaped and the size of the cross section can bebetween 1.75 mm×1.75 mm and 2.25 mm×2.25 mm.

The height range of the columns depends on the applied ultrasonicwavelength. In a specific example, a height range between the smallestand the largest column of the acoustic diffuser can be chosen between 1mm and 10 mm. For a good diffusion effect the height structure of thesecond diffuser can be chosen different from the height structure of thefirst diffuser, for example the column heights of one diffuser can bestatistically independent of the column heights of the respective otherdiffuser.

According to a further embodiment, the diffusing structure of theacoustic diffuser or the diffusing structures of the acoustic diffuserscomprises or comprise a chamber type diffusing structure, such as aquadratic residue or Schroeder diffusor or a cubic residue diffuser.

In the case of two acoustic diffusers, “the diffuser or the diffusers”can refer to one of the diffusers or to both of them. In the case ofmore than two acoustic diffusers it can refer to one of them, some ofthem or all of the diffusers. The same applies to the expression “thediffusing structure or the diffusing structures”. Furthermore, thematerial or method of production of the diffuser can also refer to thematerial or the method of production of the diffusing structure only.

According to one embodiment, the acoustic diffuser or the acousticdiffusers is made or are made from a metallic material. A metallicmaterial can be durable and provides a good acoustic coupling.

According to a further embodiment, the acoustic diffuser or the acousticdiffusers is made or are made from a plastics material. In particular,the diffuser or the diffusers can be made from a plastic material in amoulding process.

According to a further embodiment, the acoustic diffuser or the acousticdiffusers is made or are made by a 3D printing process. The 3D printingprocess is especially suitable in connection with a column-typediffuser. The 3D printing process can be a 3D metal printing process ofa 3D printing process using another type of material. The choice ofmaterial depends on the application purpose and the acoustic impedanceof the fluid, such as gas, oil, water, etc.

In particular, the cavity in which the diffuser or the diffusers areprovided can be a rectangular cavity or a cuboid shaped cavity. In thisway, the cavity can be provided by the interior space of a rectangularor cuboid shaped casing, for example.

Furthermore, the transducer can comprise a transducer element that is incontact with the emitting surface, which is provided for picking up asignal from the emitting surface. For example, this transducer elementcan be provided by a needle transducer, which is in contact with theemitting surface, or by a plate transducer, which is provided on or atthe emitting surface.

In one embodiment, the transducer comprises a needle transducer, whichextends through the cavity and unto the emitting surface. In particular,the needle transducer can extend through the excitation surface, thecavity, the diffuser or the diffuser and unto the emitting surface. Tothis end, suitable openings can be provided in the excitation surfaceand the diffusers.

The contact of the needle transducer with the emitting surface resultsin a good coupling of an acoustic wave that is received at the emittingsurface. In this embodiment, the received acoustic wave does not need totravel through the medium to the excitation surface in order to bedetected. Thereby, there is less attenuation and delay of the signal.

In particular, the needle transducer can be used for the purpose ofpicking up a sound signal from a fluid or other kind of medium that isprobed by the wide angle transducer. The needle transducer may also beused to generate an acoustic signal although preferentially the acousticsignal is generated at the excitation surface such that it is reflectedby the emitting surface, travels back and forth between the excitationsurface and the emitting surface and is diffused by the diffuser or bythe diffusers. For this purpose, a second transducer element can beprovided at the excitation surface.

Alternatively or in addition to the needle transducer, a platetransducer can be provided at the emitting surface for picking up theacoustic signal. In this case, suitable con-ducting strips, regions orcables can be provided on or at the casing which allow to connect theplate transducer to a signal processing unit.

In particular, the transducing element or the transducing elements cancomprise a piezoelectric element. In particular, some or all of thetransducer elements can be provided by piezoelectric elements. Thepiezoelectric elements can provide an effective voltage-to-soundcoupling. Moreover, piezoelectric elements can be easily available onthe market, at least when they are provided in certain standard shapes,such as plate or column shapes.

According to a further embodiment, the transducer comprises not only onetransducer element but three. Thus, transducer comprises a firsttransducer element, a second transducer element and a third transducerelement. The first transducer element is provided for picking up anacoustic signal, while the second transducer element and the thirdtransducer element are provided for generating an acoustic signal. Tothis end, the second and the third transducer elements are provided atthe excitation surface while the first transducer element is in contactwith the emitting surface.

In one embodiment, the first transducer element is arranged in a centralposition of the emitting surface. If the first transducer elementextends to the excitation surface it can furthermore be provided in acentral position of the excitation surface. The second and thirdtransducer elements can be provided as flat transducers, for example ascoin shaped transducers, which are arranged on an outer surface or anouter portion of the excitation surface.

More specifically, the first transducer element can be provided by aneedle transducer, which extends through the cavity and unto theemitting surface. As mentioned above, the needle transducer can alsoextend through the excitation surface and the diffusers. Alternatively,the first transducer element can be provided as a plate transducer atthe emitting surface.

According to one embodiment, the second transducer element and the thirdtransducer element are arranged symmetrically to the first transducerelement. This arrangement can provide well-defined conditions and canmake good use of the available space on the excitation surface. Inparticular, they can be provided symmetrical in a plane of theexcitation surface.

Moreover, the first transducer element, the second transducer elementand the third transducer element can be arranged along a diagonal of theexcitation surface. If the transducer elements are provided with astandard cross section, such as a round or a rectangular cross sectionthe cross section of the transducer elements can be made large when thetransducers are lined up along the longest extension of the excitationsurface.

According to a further embodiment, the transducer comprises twotransducer elements, one needle transducer as described above and oneflat transducer which is provided on an outer surface of the excitationsurface.

In a further aspect, the present specification discloses a measurementsystem, which comprises a computation unit with a waveform generator andthe abovementioned wide angle transducer. The computation unit isoperative to send a measuring signal from the wide angle transducer toreceive a response signal from a second transducer, and to derive ameasurement result from the response signal. More specifically, thecomputation unit and the waveform generator generate a suitably shapedelectric signal, which is transmitted by electric cable to a transducerelement of the wide angle transducer and from there to the excitationsurface.

In particular, the response signal of the other transducer can be pickedup by a transducer element that is mechanically coupled to the emittingsurface, such as the abovementioned needle transducer. The transducerelement converts the response signal into an electric signal that istransmitted to the computation unit by electric cable. The computationunit evaluates the electric signal in order to obtain the measurementresult.

According to a further type of measurement, the computation unit alsoreceives and evaluates a second response signal from the othertransducer. This second response signal corresponds to an acousticsignal that is sent from the wide angle transducer to the othertransducer in the opposite direction as the acoustic signal thatcorresponds to the first response signal.

In a further aspect, the current application discloses acomputer-implemented method for obtaining a measurement result relatingto a liquid or fluid in a conduit by means of the abovementioned wideangle transducer, wherein the conduit can be an open conduit or a closedconduit.

The liquid or fluid to be measured provided with a motion with respectto the transducer. A measuring signal is applied to one or moretransducer elements of the transducer. In particular, for theabovementioned three transducer arrangement, the measuring signal isapplied to the second transducer and to the third transducer.

Through respective mechanic coupling or contact between the transducerelement, the excitation surface and the acoustic diffuser, the measuringsignal is transmitted to the acoustic diffuser at the excitation surfaceof the transducer. In an embodiment without diffuser at the excitationsurface, the measuring signal is transmitted to the excitation surfaceand into the transmitting medium of the cavity, which is a liquid orgel.

The measuring signal is transmitted into the transmitting medium andonto the second acoustic diffuser, which is attached to the emittingsurface of the transducer. Thereby, the acoustic waves are reflectedback and forth between the first diffuser and the second diffuser and aportion of the acoustic waves is emitted at the emitting surface intothe liquid or fluid to be measured.

A response signal is received at a second transducer. The secondtransducer can be positioned with an offset relative to the wide angletransducer and to a longitudinal direction to the conduit. The secondtransducer may in particular be provided by a wide angle transducer. Ameasurement result relating to the fluid is derived from the firstresponse signal by means of a computation unit.

In the measurement arrangement, the second transducer is mounted to theconduit at a first position and the second transducer is mounted to theconduit with an offset relative to the first position with respect to alongitudinal direction of the conduit.

In a further type of measurement, the method comprises the further stepsof sending a second measuring signal, which may have the same signalshape as the first measuring signal from the second transducer to theabovementioned wide transducer.

The second response signal to the second measuring signal is received bya transducer element of the wide angle transducer, and the computationunit derives a measurement result relating to the fluid from the firstresponse signal and the second response signal.

In a further aspect, the current application discloses a method forproducing a transducer. A casing is provided, which is made from aplastic material or a metal material. A first diffuser is produced by a3D metal printing process. The first diffuser is provided at an emittingsurface of the casing.

Furthermore, a second diffuser can be produced by the 3D printingprocess and provided at an excitation surface of the casing, theexcitation surface being opposite to the emitting surface. Inparticular, the first diffuser and/or the second diffuser can beprovided on the respective surface by gluing, welding, soldering,screwing, riveting or by manufacture in one piece with the casing.

A transducer element is provided at the excitation surface of thecasing. For example, the transducer element or the transducer elementscan be provided by gluing, welding, soldering and/or a mechanical fit,such as press fit or form fit. Furthermore, a second transducer can beprovided in contact with the emitting surface or at the emitting surfacefor picking up an acoustic signal.

Furthermore, the present specification discloses a transducerarrangement of at least two wide-angle transducers for performing a flowspeed measurement. Specifically, the present specification discloses atransducer arrangement of at least two wide-angle transducers forperforming a flow speed measurement using a time reversed signal.

More specifically, a transducer arrangement is disclosed that comprisesa first wide-angle transducer according to one of the embodiments and asecond wide-angle transducer according to one of the embodiments, asignal generating unit and a signal processing unit. The signalgenerating unit and the signal processing unit are realized withelectronic components, such as circuits and integrated circuits.

The signal generating unit is connected to the first transducer and tothe signal processing unit, and the signal processing unit is connectedto the second transducer.

The first transducer, the second transducer, the signal generating unitand the signal processing unit are configured to apply a predeterminedfirst signal to the first transducer, such as a time focused impulsesignal, to receive a response signal of the predetermined first signalat the second transducer, to derive a measuring signal from the responsesignal, the derivation of the measuring signal comprising selecting asignal portion of the response signal or of a signal derived therefromand reversing the signal portion with respect to time, to store themeasuring signal for later use in an electronic memory component, toapply the measuring signal to the first transducer, to receive aresponse signal to the measuring signal at the second transducer, and toderive a flow speed from the received response signal to the measuringsignal.

In a further embodiment, which uses a two-way flow speed measurement,which can be used to cancel out a temperature dependence, the signalgenerating unit is furthermore connected to the second transducer andthe signal processing unit is furthermore connected to the secondtransducer.

The first transducer, the second transducer, the signal generating unitand the signal processing unit are configured to apply the measuringsignal to the second transducer, to receive a second response signal tothe measuring signal at the first transducer, to derive a flow speedfrom the received response signal to the measuring signal at the secondtransducer and from the received second response signal to the measuringsignal at the first transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present specification is now explained infurther detail with respect to the following Figures, in which:

FIG. 1 shows a side view of a first embodiment of a wide angle acoustictransducer with two plate shaped transmitter elements and a columnshaped receiver element,

FIG. 2 shows a perspective view of an acoustic diffuser for an acoustictransducer,

FIG. 3 shows a perspective view of a further wide angle acoustictransducer with a single transducer element and two diffusers,

FIG. 4 shows a perspective view of a further wide angle transducerhaving three transducer elements and two diffusers,

FIG. 5 shows a perspective view of a further wide angle acoustictransducer with one plate transducer element, a needle transducer andtwo diffusers,

FIG. 6 shows an experimental setup for a pressure distributionmeasurement of an acoustic transducer,

FIG. 7 shows a pressure distribution of an acoustic transducer with arectangular cavity,

FIG. 8 shows a reference pressure distribution of an acoustic transducerhaving a cavity with a diffuser having a small column size,

FIG. 9 shows a pressure distribution of an acoustic transducer having acavity with a diffuser having a larger column size,

FIG. 10 shows a reference pressure distribution of an acoustictransducer having a rectangular cavity,

FIG. 11 shows a pressure distribution of an acoustic transducer with twodiffusers facing each other,

FIG. 12 shows an arrangement of clamp-on transducers for use with theembodiments of FIGS. 1 to 11,

FIG. 13 shows a further arrangement of clamp-on transducers for use withthe embodiments of FIGS. 1 to 11 in a V-configuration measurement,

FIG. 14 shows the arrangement of FIG. 13 in a W-configurationmeasurement,

FIG. 15 shows a further arrangement of clamp-on transducers for use withthe embodiments of FIGS. 1 to 11,

FIG. 16 shows a received signal in the arrangement of FIG. 12, and

FIG. 17 shows an arrangement of wet transducers for use with theembodiments of FIGS. 1 to 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, details are provided to describe theembodiments of the present specification. It shall be apparent to oneskilled in the art, however, that the embodiments may be practisedwithout such details.

FIG. 1 shows a cross sectional view of a wide-angle transducer 10according to a first embodiment. The wide-angle transducer 10 comprisesa casing 11, which encloses a cavity 12. A first plate shaped transducerelement 13 and a second plate shaped transducer element 14 are providedon an excitation surface 15 of the casing 11. The transducer elementshave connections for connecting the transducer elements to an electricpower source, which are not shown in FIGS. 1, 3, 4 and 5.

In particular, the transducer elements of this embodiment and of thefollowing embodiments can be provided by piezoelectric ceramics.According to specific embodiment, which was used to obtain the pressurecurves, one or more 1 MHz piezoelectric discs with a diameter of 2 cmare glued to the flat back of the acoustic diffuser with a thin layer ofepoxy. Silicone gel is applied to the other side of the piezoelectricdisc for electrical insulation.

An acoustic diffuser 16 is provided on the inside of an emitter surface17 of the casing 11, which is opposite the excitation surface 15. Thediffuser 16 is shaped such that it provides a diffuse reflection of asoundwave emitted by the transducer elements 13 and 14. Furthermore, thediffuser 16 is shaped such that the acoustic modes of the cavity 12,which would otherwise lead to standing waves within the cavity 12, aresuppressed. In the example of FIG. 1, the diffusor 16 is a chamber typediffusor, such as a Schroeder diffuser.

Examples of such diffusers include, among others, a Schroeder diffuseror quadratic residue diffuser (QRD), a cubic residue diffuser (CRD), aprimary root diffuser (PRD) and a column type diffuser withstatistically distributed heights. In one embodiment, which isparticularly suitable for 3D printing, the diffuser is a column typediffuser with rectangular columns that have statistically distributedheights.

In the example of FIG. 1, the cavity 11 has the shape of a rectangularblock, which is bounded by side walls 15, 17, 18 of the casing 11. Theside walls 15, 17, 18 of the casing 11 comprise the excitation surface15, the emitter surface 17, a first lateral surface 18, a second lateralsurface 19 and third and fourth lateral surfaces which are not shown inFIG. 1.

A needle transducer 20 of the wide angle transducer 10 is coupled to theemitting surface 17 and extends through the cavity 17 and the excitationsurface 17 and protrudes from the excitation 15 surface towards theexterior of the casing 11. In the example of FIG. 1, the needletransducer 20 is provided in a central position of the emitting surface17 and the needle transducer 20 extends through the emitting surface 17.

In other embodiments, the needle transducer 20 can be coupled to theemitting surface 17 in other ways. For example, the needle transducermay be welded or glued to an inner surface of the emitting surface 17.Alternatively, the emitting surface 17 may comprise a reception portionwith a suitable shape for taking up the needle transducer 20.

The cavity 12 is filled with an amorphous sound transmitting medium 21such as a liquid or a gel or epoxy or other relevant material.Preferentially, the speed of sound in the transmitting medium is greaterthan the speed of sound in air and smaller than the speed of sound inthe casing 11. Furthermore, the sound attenuation and thecompressibility of the sound transmitting medium are substantiallysmaller than the sound attenuation and the compressibility of air.

In many materials, the sound attenuation depends on the dynamicviscosity, the bulk viscosity of the material and the sound frequency,whereas the speed of sound depends on the compressibility of thematerial. However, the sound attenuation of sound transmittingmaterials, such as the sound transmitting medium 21 may also becharacterized by the compressibility of the material. In order to avoidcorrosion of the interior space of the wide angle transducer, thetransmitting medium can be provided by a non-corrosive fluid such asoil.

FIG. 2 shows a further embodiment of an acoustic diffuser 16′ theacoustic diffuser is provided by an array of columns 25, which havestatistically distributed heights with respect to a base portion 26.Each pillar or column 25 has a square cross-section and is connected tothe base portion platform 26, which has a rectangular shape. The arrayof columns is also known as a “Manhattan”, “terrain” or “skyline”structure. By way of example, the statistical height distribution may begenerated by a random sequence generator. The range of random numbers isrestricted between a minimum and a maximum height, which is sufficientto provide a diffuse reflection.

The acoustic diffuser 16′ of FIG. 2 is a 3d-printed acoustic diffuserconstructed with multiple pillars of different length. The acousticdiffusor comprises closely packed metal pillars with a random height.Each pillar has square cross-section, and is connected to a platform ofrectangular shape.

The random sequence that determines the height distribution may be apseudo random sequence generated by an algorithm or it may be a realrandom sequence generated by a hardware random number generator using aquantum mechanical measurement such as shot noise or some other physicalrandom process such as thermal noise. For example, the random heightscould be selected according to a uniform distribution, a Gaussiandistribution, a Poisson distribution or some other form of statisticaldistribution.

In a terrain design of an acoustic diffuser the upper and lowerfrequency limits are defined by the width and height of the squarecolumns, respectively. The height of the columns defines the lowerfrequency boundary, while the upper frequency boundary is defined by thewidth of the column. In specific examples, the height range of thecolumns or pillars may be chosen between 1 mm and 10 mm, the dimensionsof the base surface or “foot print” may be chosen as 3.8 cm×3.8 cm, andthe column width may be chosen as 1 mm or 2 mm. Thereby, the diffuserhas an array of 38×38=1444 columns or, respectively, of 19×19=361columns.

The acoustic diffuser 16, 16′ is preferentially made from a metallicmaterial, which provides a good coupling to the emitting surface. Inparticular, the diffuser 16, 16′ can be made by a 3D metal printing.Suitable processing for 3D metal printing include, among others, metalsintering or melting and in particular selective laser sintering (SHS),direct metal laser sintering (DMLS), selective laser melting (SLM),electron beam melting (EBM), powder bed and inkjet head 3D printing(3DP), Fused deposition modelling (FDM) or Fused Filament Fabrication(FFF), Robocasting or Direct Ink Writing (DIW) and electron beamfreeform fabrication (EBF3).

The current 3D printing technology is able to print structure as smallas tens of micron in a size suitable for high frequency time reversalacoustics applications. It is suitable to manufacture complex surfacestructures. For a mass production molds could be used and the structurecould be hot pressed or molded. The wide angle diffusor that was usedfor the experiments may be produced as follows.

First, a volumetric model of the acoustic diffuser is generated with acomputational geometry programming language, such as OpenSCAD. By way ofexample, the wide angle transducer of the pressure measurements has asupport surface of 3.8 cm×3.8 cm which is decorated with verticalpillars of square cross-section, which is 1 mm by 1 mm and 2 mm by 2 mmwide. The height of the columns is generated randomly. For the diffuserof the pressure measurement, a uniform distribution with values rangingbetween 1 mm and 10 mm was used. For printing, the geometry may beexported in a standard file format, such as STL, and is converted into aprintable file, which is suitable for printing with a 3D metal printer.

By way of example, the 3D metal printer for generating the diffuser thatwas used in the pressure measurements utilizes a 100 W infrared fibrelaser with a focus diameter of 40 micron to sinter austenitic stainlesssteel powder of 50 micron.

FIG. 3 shows another embodiment of a wide angle transducer 10′. The wideangle transducer 10′ comprises two acoustic diffusers 16′, 36 facingeach other. The acoustic diffusers 16 and 36 are column type diffusers,as shown in the previous FIG. 2. For the sake of clarity, FIGS. 3 and 4show only a portion of the second diffuser 36. The second diffusor 36can be seen best in the cross sectional view of the embodiment of FIG.5. For brevity, some explanations of features which are common to theembodiments are not repeated again in the description of FIGS. 3, 4 and5.

The first acoustic diffuser 16′ is provided at the emitting surface 17of the wide angle transducer 10′. The second acoustic diffuser 36 isprovided at the excitation surface 36, which is opposite to the emittingsurface 17. Different from the embodiment of FIG. 1, the wide angletransducer 10′ comprises only a single plate transducer 20′ which isused both for emitting and for receiving acoustic waves.

In a transmitting mode, the plate transducer 20′ is supplied with anelectric signal and converts the electric signal into an acousticsignal, which is transmitted to the excitation surface 15 and to thesecond diffuser 36. From there, the signal is transmitted into themedium 21 and to the first diffuser 16′. A portion of the signal isemitted at the emitting surface 17 and another portion is reflected backinto the medium 21.

The reflected acoustic signal is reflected back and forth between thefirst diffuser 16′ and the second diffuser 36 until it is dissipated.Every time the first diffuser 16 receives an acoustic signal it emitspart of it at the emitting surface 17. The emitting surface 17 is eitherin direct contact with a fluid or is coupled to it and emits theacoustic wave into the medium. In the context of the presentspecification, a fluid can be a liquid like oil, water, mixtures thereofor also a gas like oil, petroleum gas or mixture thereof. Generallyspeaking, a leaky cavity is formed, which is driven by one or severaltransducer elements, such as piezoelectric ceramics, which are attachedto the flat back of one of the two diffusers 16, 36.

In a receiving mode, the plate transducer 20′ receives an acousticsignal, which is transmitted from the emitting surface 17 to thediffuser 16 and into the medium 21. Furthermore, the plate transducer20′ receives signal portions that have been reflected back and forthbetween the diffuser 16, the walls of the casing 11 and the seconddiffuser 36.

FIG. 4 shows a further embodiment of a wide angle transducer 10″ inwhich three plate transducers 13, 14, 20′ are provided in a row along adiagonal of the excitation surface 15. One of the transducers 20′, whichis used as a transmitter, is provided centrally on the excitationsurface 15. The two other transducers 13, 14 are provided on either sideof the transducer 20′.

FIG. 5 shows a cross sectional view of a wide angle transducer 10, whichis similar to the transducer 10 of FIG. 1 in that the centrally arrangedtransducer element 20 is provided as a needle transducer 20. In FIG. 5,the needle transducer 20, the diffuser 16 and the casing 11 are shown incross section.

Similar to the embodiments of FIGS. 3 and 4 and different from theembodiment of FIG. 1, two column type diffusers 16, 36 are provided atopposite sides of the cavity. Similar to the embodiment of FIGS. 1 and 4and different from the embodiments of FIG. 3 there are three transducerelements 13, 14, 20, which are arranged along a diagonal of theexcitation surface 15. The first transducer element is not shown in theview of FIG. 5. In principle, the transducer elements 13, 14, 20′ couldbe arranged in other patterns but the pattern of FIG. 4 allows toachieve a large surface of circular shaped transducer elements and aneffective coupling to the excitation surface 15.

FIG. 6 shows a schematic configuration of an experimental setup that isused for determining the pressure distribution of a wide angletransducer 30 according to FIGS. 7 to 11. The wide angle transducer 30refers to one of the wide angle transducer 31, 32, 33, 34 shown in theinsets of FIGS. 7, 8, 9, 10 and 11, respectively. The acoustic diffuser30 is submerged in water 37 in an upright position.

The water 37 is contained between the boundaries of a container or aduct, which are indicated by the boundary lines 28 and 29. For thepurposes of this measurement, the water is at rest and is not flowingrelative to the boundaries 28, 29.

A piezoelectric ceramic 20′ is attached to the wide angle transducer 30and is backed by air 38 in order to have an efficient coupling of thesignal into the water 37. A needle hydrophone 39, which is arranged atthe boundary 29 in upright position and opposite to the wide angletransducer 37 is used to measure the acoustic pressure. The coordinatesare chosen at water surface with the origin placed at the center of thepiezoelectric ceramic. The X- and Y axes are lateral to the surface ofthe transducer and the Z-axis increases with normal distance from thetransducer 30. For the purpose of the pressure diagrams of FIGS. 7 to11, only the X-coordinate, which is parallel to the boundary 29, isused.

The needle hydrophone 39 is moved in steps along the X-coordinate topick up the sound signal from the wide angle transducer 30. In FIG. 6,the needle hydrophone 39 is shown in a position opposite to the centreof the wide angle transducer 30. As mentioned further below, a distancebetween the hydrophone and the front side of a plate, which serves as anemitting surface, is 150 mm.

The acoustic diffusors are mounted inside a plastic housing, whichallows inserting one or two of them. By way of example, this is shown inFIGS. 3, 4 and 5. The housing is in turn connected to a stainless steelrod and fixed in place at the free surface of a water filled basin madeof transparent acrylic, see FIG. 6.

In a technical application, the boundary 28 will usually be provided bya wall, such as a conduit wall. In an open channel configuration, thetransducer is provided at a free surface. For the purpose of measuringonly one direction of the acoustic signal and for ease of placement ofthe wide angle transducer it is sufficient to measure in an open channelconfiguration.

By placing the transducer at the water surface, the piezo element 20′becomes air-backed which increases the pressure amplitude transmittedinto water. The signal is generated with an arbitrary waveformgenerator, fed to 55 dB RF amplifier (350 MHz bandwidth, ENI) andconnected directly to the piezoelectric element 20′.

For measurements of the spatial pressure distribution a single cycle at1 MHz with a peak-to-peak voltage of 80 Volt is applied to the piezoelement 20′. The acoustic signals are measured with a small hydrophone39 with a circular polyvinylidenefluoride (PVDF) sensor. While theemitter/receiver is at a fixed location the position of the hydrophone39 is moved with a programmable translation stage. When scanning thesound field the hydrophone is translated in steps of 0.5 mm where y=0, xis varied between −100 mm and 100 mm and z=150 mm.

The measurement is automated. After the positioning of the hydrophone39, the electric signal is generated from a control computer, uploadedto the arbitrary waveform generator, triggered and then captured with a14-bit sampling oscilloscope. The signals are then transferred to thecomputer and stored.

To investigate the impact on the decoration of the plate on the spatialscattering, the spatial pressure distribution of two different terrainstructures were studied. A flat plate of thickness 6 mm served as acomparison.

The FIGS. 7, 8 and 9 show the spatial distributions of the maximumpositive pressure measured at a distance of 150 mm from the front sideof the plate for three distinct configurations: a flat plate in FIG. 7,a diffuser with a terrain structure with 1 mm pillar size in FIG. 8, anddiffuser with a terrain structure with 2 mm pillar size in FIG. 9.

FIG. 7 shows a reference pressure distribution created by a wide angletransducer having just a cavity and no diffuser. As can be seen in FIG.7, the flat plate provides a symmetric and narrow beam with a singledistinct peak having a full width at half maximum (FWHM) of 26 mm.

FIG. 8 shows a pressure distribution created by a wide angle transducerhaving a single column type diffusor with a smaller column base size of1 mm by 1 mm. In the arrangement of FIG. 8 there is a strong differenceon the spatial pressure distribution using the terrain structure of 1 mmpillar size as compared to FIG. 7. Several side lobes are prominentlyvisible. Yet the FWHM of the main peak at 25 mm, has not significantlychanged.

FIG. 9 shows a pressure distribution created by a wide angle transducerhaving a single column type diffusor with a larger column base size of 2mm by 2 mm. The pressure distribution of FIG. 9, which was obtained witha diffuser having a terrain with 2 mm pillar size is significantlydifferent. In particular, the distribution in the centre region isdifferent, it appears to consist of three peaks of approximately similaramplitude spanning around 80 mm at FWHM. This measurement suggests thatthe terrain diffuser with the larger pillar size will perform betterwithin the leaky cavity.

For the pressure measurements of FIGS. 10, 11 and 12, a reverberationcavity was constructed. Thereby, the acoustic diffuser can also be usedas a receiver.

To obtain a compact design, two parallel diffuser plates are embeddedinto the cavity with their terrain surface facing each other. An ABSmade plastic holder fixes and aligns the plates such that there remainsa gap between the tallest pillars. This gap is filled with water.However, in other embodiments the cavity can be filled with any otheracoustic transparent material, such as a gel etc.

Care is taken that no gas bubbles become enclosed, which could lead toundesired effects, such as cavitation. Therefore, prior to assembly thediffusers are kept in water under low pressure inside a vacuum chamberand the water is degassed. The diffusers are assembled into the cavitywhile remaining submerged. The final structure of the transducer, namelythe pillar cavity, has a dimension of 4 cm×4 cm×2.5 cm. For referencepurposes, the performance is performed with a cavity formed by twoundecorated plates of 6 mm thickness each, which is also referred to as“flat cavity”.

During the pressure measurements, the large mismatch in acousticimpedance between steel and water results in a high Q-value of thecavity, which is needed for the desired long reverberation times. Inother words, a high density contrast leads to a high reflection rate.

In FIGS. 10 and 11, the pressure distribution of the respective cavitiesis measured to confirm a wide spatial emission. As before in themeasurements of FIGS. 7, 8 and 9, a single cycle 1 MHz signal is usedbut the scan range is increased to −125 mm≤×≤125 mm and z=150 mm of thehydrophone. The measured distribution of the maximum pressure emittedfrom the cavities is depicted in FIGS. 10 and 11.

FIG. 10 shows a reference pressure distribution created by a wide angletransducer having a cavity which is bounded by two flat or undecoratedplates 40, 41. The FWHM of the flat cavity of FIG. 10 is about 33 mm.

FIG. 11 shows a pressure distribution created by a wide angle transducersimilar to the one shown in FIG. 3, which has diffuser plates 16′, 36that are facing each other. Compared to the FWHM of the pressuredistribution of FIG. 10, the terrain cavity of FIG. 11 displays a nearly5-fold increased FWHM of 145 mm. Thus, the FWHM extension is much largerthan the width of the diffuser base, which is 3.8 cm=38 mm. Overall, thecavity transducer with the terrain surfaces greatly enhance the spatialspreading while maintaining a compact form.

An opening angle α of the trapezoid, which is formed between thepressure FWHM as first base line and the plate surface as second baseline, may be defined by

$\alpha = {{\sin^{- 1}( {\frac{{145\mspace{14mu} {mm}} - {38\mspace{14mu} {mm}}}{2}*\frac{1}{150\mspace{14mu} {mm}}} )} \approx {20.9{^\circ}}}$

Generally, the time resolution will be limited by the damping time ofthe cavity.

FIG. 12 shows an arrangement of two clamp-on transducers 10. Thearrangement of FIG. 12 and the arrangements of the following FIGS. 13,14, 15, 17 can be used with the wide-angle transducers 10, 10′, 10″,10′″ of FIGS. 1 to 11.

A flow direction is indicated by a horizontal arrow and travel paths ofacoustic signals is indicated by two diagonal arrows.

FIG. 13 shows a further arrangement of clamp-on transducers 110 for usewith the embodiments of FIGS. 1 to 11 in a V-configuration measurement.The wide-angle transducers 10-10′″ can be integrated into a clamp-ontransducer 110 in various ways. For example, the wide-angle transducers10-10′″ can be arranged such that an excitation surface 15 coincideswith an outer surface of the clamp-on transducer 110 and the oppositeemitting surface 17 is adjacent to a wedge-shaped part of the clamp-ontransducer 110.

FIG. 14 shows the arrangement of FIG. 13 in a W configurationmeasurement in which a dominant acoustic signal is reflected twice atthe boundaries of a conduit before it reaches a second transducer.

FIG. 15 shows a further arrangement of clamp-on transducers for use withthe embodiments of FIGS. 1 to 11. In the example of FIG. 15 thewide-angle transducers 10 are fitted in to the clamp on transducers,which are clamped to a conduit by means of cables. In the arrangement ofFIG. 15, is one set of four transducers, one intermediate set of twotransducers and a further set of four transducers. Lines of sightbetween the wide-angle transducers 10 are indicated by dashed lines.

FIG. 16 shows a received signal in the arrangement of FIG. 12. Thesignal FIG. 15 illustrates the focusing properties of a wide-angletransducer in the time domain. A focusing in the time domain can beachieved by applying a time inverted signal at the sending wide-angletransducer 10.

A measuring signal according to the present specification can bemodelled by a matched filter. If a sharply peaked impulse is used as aprobe or test signal, the received signal at the transducer is theimpulse response of the channel. The wide-angle transducers of thepresent specification can be used with or without time inverted signals.When a time inverted signal is used, an inverted version of a receivedsignal is sent back through the same channel as a measuring signal,either in the reverse direction of a flow in a conduit or in the samedirection as the flow.

This results in a signal with a peak at the origin, where the originalsource was, or in a signal with a peak at the original receiver,respectively. The signal is shaped with respect to space and time. Thetime focussing property is illustrated in FIG. 16, while the spatialshaping is illustrated in FIGS. 8, 9 and 11.

An ultrasonic flow meter according to the present specification canprovide a focusing property by using the abovementioned inverted signal,or a similarly shaped signal, for an ultrasonic flow meter to form aresponse signal, which is both concentrated in space and time. This inturn leads to a higher amplitude at a receiving piezoelectric elementand a better signal to noise ratio.

With an ultrasonic flow meter according to the present specification,focusing and beam forming properties can be obtained under very generalconditions. For example, a focusing property is obtained even when onlyone ultrasound transmitter is excited and even when the inverted signalis reduced to signal that is only coarsely digitized in the amplituderange, if the time resolution of the inverted signal is sufficient.Furthermore, a flow meter according to the present specification can beused with clamp-on transducers, which are easy to position on a pipe anddo not require modifications of the pipe.

FIG. 17 shows an arrangement of wet transducers 111 for use with theembodiments of FIGS. 1 to 11. The wet transducers 111, which protrudeinto the conduit, comprise wide angle transducers 10. In the example ofFIG. 17 there are altogether five layers of four transducers. Diagonallines of sight within the same layer have been indicated by dashedlines. This arrangement is particularly useful for measuring the flowvelocity in the different layers separately.

The arrangement can be best used in combination with wide-angletransducers of the present specification and beam shaping procedures.The beam shaping procedures are implemented by electronic componentssuch as a signal generating unit and a signal evaluation unit, which isalso known as a signal processing unit. In particular, the beam shapingmay comprise the application a time inverted signal to a wide-angletransducer.

Although the above description contains much specificity, these shouldnot be construed as limiting the scope of the embodiments but merelyproviding illustration of the foreseeable embodiments. Especially theabove stated advantages of the embodiments should not be construed aslimiting the scope of the embodiments but merely to explain possibleachievements if the described embodiments are put into practise. Thus,the scope of the embodiments should be determined by the claims andtheir equivalents, rather than by the examples given.

REFERENCE

-   10 wide angle transducer-   11 casing-   12 cavity-   13 transducer element-   14 transducer element-   15 excitation surface-   16, 16′ acoustic diffuser-   17 emitting surface-   18 first lateral surface-   19 second lateral surface-   20 transducer element-   20′ needle transducer-   21 transmitting medium-   25 columns-   26 base surface-   28 boundary line-   29 boundary line-   30 wide angle transducer-   31 reference transducer-   32 wide angle transducer, small column size-   33 wide angle transducer, large column size-   34 reference transducer-   35 wide angle transducer, double sided-   36 acoustic diffuser-   37 water-   38 air-   39 needle hydrophone-   40 flat plate-   41 flat plate-   110 clamp-on transducer-   111 wet transducer

What is claimed is:
 1. A transducer for emitting and receiving acousticwaves, the transducer comprising a casing, the casing forming a cavity,the casing comprising: an excitation surface and an emitting surfacewhich is arranged opposite to the excitation surface, a transducerelement, the transducer element being provided at the excitationsurface, an acoustic diffuser, the acoustic diffuser being provided atthe emitting surface of the casing, wherein a diffusing structure of theacoustic diffuser faces the cavity, and wherein the diffusing structureis provided by an array of columns.
 2. The transducer according to claim1, wherein the columns of the array of columns have a statisticallydistributed height.
 3. The transducer according to claim 2, wherein aheight distribution of the statistically distributed height is generatedby a random sequence generator
 4. The transducer according to claim 1,wherein the columns of the array of columns are aligned orthogonal to abase surface of the acoustic diffuser.
 5. The transducer according toclaim 1, wherein a height range of the columns is between 1 millimeterand 10 millimeters.
 6. The transducer according to claim 1, thetransducer comprising a second acoustic diffuser, the second acousticdiffuser being provided at the excitation surface, wherein a diffusingstructure of the second acoustic diffuser faces the cavity.
 7. Thetransducer according to claim 6, wherein the diffusing structure of thesecond acoustic diffuser faces the diffusing structure of the firstacoustic diffuser.
 8. The transducer according to claim 1, wherein theacoustic diffuser is made from a metallic material.
 9. The transduceraccording to claim 1, wherein the acoustic diffuser is made from aplastic material.
 10. The transducer according to claim 1, wherein thecavity is a rectangular cavity.
 11. A transducer for emitting andreceiving acoustic waves, the transducer comprising a casing, the casingforming a cavity, the casing comprising an excitation surface and anemitting surface which is arranged opposite to the excitation surface, atransducer element, the transducer element being provided at theexcitation surface, an acoustic diffuser, the acoustic diffuser beingprovided at the emitting surface of the casing, wherein a diffusingstructure of the acoustic diffuser faces the cavity, the transducercomprising a first transducer element, a second transducer element and athird transducer element, the second transducer element and the thirdtransducer element being provided at the excitation surface and thefirst transducer element being in contact with the emitting surface. 12.The transducer according to claim 11, wherein the first transducerelement is a plate transducer, the plate transducer being provided atthe emitting surface.
 13. The transducer according to claim 11, whereinthe first transducer element is arranged in a central position of theemitting surface.
 14. The transducer according to claim 11, wherein thesecond transducer element and the third transducer element are arrangedsymmetrically to the first transducer element.
 15. The transduceraccording to claim 13, wherein the first transducer element, the secondtransducer element and the third transducer element are arranged along adiagonal of the excitation surface.
 16. A transducer for emitting andreceiving acoustic waves, the transducer comprising a casing, the casingforming a cavity, the casing comprising an excitation surface and anemitting surface which is arranged opposite to the excitation surface, atransducer element, the transducer element being provided at theexcitation surface, an acoustic diffuser, the acoustic diffuser beingprovided at the emitting surface of the casing, wherein a diffusingstructure of the acoustic diffuser faces the cavity, the transducercomprising a plate transducer that is provided on or at the emittingsurface.
 17. The transducer according to claim 16, the transducercomprising a first transducer element, a second transducer element and athird transducer element, the second transducer element and the thirdtransducer element being provided at the excitation surface and thefirst transducer element being in contact with the emitting surface. 18.The transducer according to claim 17, wherein the first transducerelement is arranged in a central position of the emitting surface. 19.The transducer according to claim 17, wherein the second transducerelement and the third transducer element are arranged symmetrically tothe first transducer element.
 20. The transducer according to claim 19,wherein the first transducer element, the second transducer element andthe third transducer element are arranged along a diagonal of theexcitation surface.